At 03:45 PM 6/18/2011, David wrote:

>I wish to drive a IR LED at 40khz (essentially the Sony IR protocol),
>somewhere between 50mA and 100mA max LED Ifwd.  In testing I have been
>using a 2N7000.
>
>I am confused around whether or not to use resistors, there is a lot of
>conflicting advice online.  The options seem roughly to be:

Use a series resistor mounted right at the FET if you have long leads=20
on either the gate or source.  Reason: the FET will want to oscillate=20
at very high frequency and that is destructive if you are switching=20
large amounts of power.

If the FET is mounted within a small distance from the PIC, series=20
resistor probably not needed.

Use a pull-down resistor if there is any chance that the PIC pin that=20
drives the FET will ever be floating for a significant amount of=20
time.  Example: pulling the PIC out of its socket for re-programming=20
while power is still applied.

If you can guarantee that the FET gate won't ever be left floating,=20
the pull-down resistor can be omitted.

dwayne

--=20
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

--=20
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